1. The word "guest" suggests that the Soul (notice the capitalization of soul) is an essential being that is actually separate and independent of the body. As the poem continues, it is further suggested that the Soul holds key truths about humanity that humans themselves refuse to accept, preferring to accept a lie. This idea of the preference for the lie is continued in line three, when the Soul's quest for the illumination of people is referred to as being thankless. The idea that the soul is independent of the body is furthered at the end of the stanza when Ralegh refers to the Soul as leaving his body and continuing its errand.
2. Errand (The New Oxford Book of Sixteenth-Century Verse)
3. Potentate, according to the Oxford English Dictionary, refers to an individual with independent power (i.e. a prince, monarch, or ruler).
4. Passion or lust (New Oxford Book)
5. Ambition, again according to the OED, is a desire to attain a higher position or rank, but inordinately. Hence, Ralegh's view of the "men of high condition" and their ambition is not a complement of their ambitious attitudes, but a criticism of their greed.
6. Brave, instead of an adjective meaning courageous, is here used as a verb meaning to challenge or defy (OED). However, the use of the word "it" is somewhat confusing and unspecific, leaving the reader somewhat lost as to what the "it" is referring to.
7. Zeal here refers to an ardent love with possible negative implications leading to rivalry, jealousy, and partisanship (OED). Again Ralegh is trying to convince the reader that what we hold to be true and good are in fact false and therefore negative. And if any one dares refuse this truth, the soul (in this last stanza referred to as "thou") gives the lie, allowing the hopeless, naive humans to stay in the dark.
8. Measures (New Oxford Book)